U.S. patent number 6,048,511 [Application Number 09/225,820] was granted by the patent office on 2000-04-11 for method for forming high density boron nitride and high density agglomerated boron nitride particles.
This patent grant is currently assigned to Advanced Ceramics Corporation. Invention is credited to Richard Frank Hill, Gregory W. Shaffer.
United States Patent |
6,048,511 |
Shaffer , et al. |
April 11, 2000 |
Method for forming high density boron nitride and high density
agglomerated boron nitride particles
Abstract
A method of forming pellets or agglomerates of high density
boron nitride from high purity hexagonal boron nitride by crushing
the high purity hexagonal boron nitride into boron nitride
particles extending over a size range of at least 100 microns with
the majority of the particles having a particle size above 50
microns and cold pressing the crushed particles into a compacted
form. The compacted form is then granulated into a granulated
powder and again cold pressed to form pellets or agglomerates of
boron nitride particles with the operations of cold pressing and
granulation occurring in one or more stages.
Inventors: |
Shaffer; Gregory W.
(Strongsville, OH), Hill; Richard Frank (Chargrin Falls,
OH) |
Assignee: |
Advanced Ceramics Corporation
(Lakeland, OH)
|
Family
ID: |
24477384 |
Appl.
No.: |
09/225,820 |
Filed: |
January 5, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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968959 |
Nov 12, 1997 |
5898009 |
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618361 |
Mar 19, 1996 |
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Current U.S.
Class: |
423/290;
264/117 |
Current CPC
Class: |
C04B
35/5831 (20130101); C08K 3/38 (20130101); C01B
21/064 (20130101); C01B 21/0648 (20130101); C01P
2004/45 (20130101); C01P 2004/50 (20130101); C01P
2006/10 (20130101); C01P 2006/12 (20130101) |
Current International
Class: |
C04B
35/583 (20060101); C04B 35/5831 (20060101); C08K
3/00 (20060101); C08K 3/38 (20060101); B29C
067/02 (); C01B 021/064 () |
Field of
Search: |
;423/290 ;501/96.4
;264/65,117,118 |
References Cited
[Referenced By]
U.S. Patent Documents
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5064589 |
November 1991 |
Ichikawa et al. |
|
Primary Examiner: Langel; Wayne
Attorney, Agent or Firm: Lieberstein; Eugene Meller; Michael
N.
Parent Case Text
This application is a division of application Ser. No. 08/968,959
filed Nov. 12, 1997, Pat. No. 5,898,009, which is a continuation of
application Ser. No. 08/618,361 filed Mar. 19, 1996, abandoned.
Claims
What we claim is:
1. A method of forming pellets or agglomerates consisting of high
density boron nitride comprising the steps of forming high purity
hexagonal boron nitride; crushing said high purity hexagonal boron
nitride into boron nitride particles extending in size over a size
range of at least 100 microns with the majority of the particles
having a particle size above 50 microns, cold pressing the crushed
particles into a compacted form and granulating the compacted
particles into a granulated powder with the operations of cold
pressing and granulation occurring in one or more stages for
forming pellets of boron nitride or agglomerates of high density
boron nitride.
2. A method as defined in claim 1 further comprising the additional
step of crushing the compacted boron nitride into a multiplicity of
particles which vary in size over a wide particle size distribution
with a majority of particles lying in the size range of between
20-500 microns and agglomerating the boron nitride particles for
use as a filler.
3. A method as defined in claim 1 wherein the majority of the
particles have an average particle size above 200 microns.
4. A method as defined in claim 3 wherein said particle size range
extends from below 20 microns to above 400 microns.
5. A method as defined in claim 4 wherein said particle size range
extends up to 500 microns.
6. Boron nitride pellets consisting of boron nitride having a
density of between 1.86 and 1.91 g/cc for use in converting
hexagonal boron nitride into cubic boron nitride formed by the
process comprising: crushing high purity hexagonal boron nitride
into a multiplicity of boron nitride particles which vary in size
extending over a particle size range of at least 100 microns with
the majority of the particles above 50 microns in size, cold
pressing the crushed particles into a compacted form, granulating
the compacted particles into a granulated powder and cold pressing
the granulated powder into pellets with the operations of cold
pressing and granulation occurring in one or more stages.
Description
FIELD OF THE INVENTION
The present invention relates to a method for forming high density
boron nitride for use as precursor feedstock material in the
conversion of hexagonal boron nitride to cubic boron nitride and as
high density agglomerated boron nitride particles formed from such
method for use as high thermal conductivity fillers.
BACKGROUND OF INVENTION
The two conventional forms of hexagonal boron nitride are
turbostratic boron nitride and ideal or graphitic boron nitride.
The hexagonal form of boron nitride is used in the conversion to
cubic boron nitride and as a filler material for many other
applications particularly where high thermal conductivity and high
electrical resistivity is required. Typically, turbostratic boron
nitride is first purified into what is conventionally referred to
as "high purity hexagonal boron nitride" by treatment at high
temperature, typically between about 1800.degree. C. to
1900.degree. C., for removing volatile impurities and surface oxide
contaminants. Such high temperature treatment causes the boron
nitride to become highly agglomerated in consistency which must be
broken down for suitable commercial application. Accordingly,
current practice is to first mill the high purity boron nitride
into a fine powder and then, for ease of handling, to cold press
and granulate the boron nitride in one or more stages. The milling
operation forms a fine powder of small particle size typically with
99.9% of all of the milled powder below -325 mesh. The average
particle size of the milled powder lies between 5-11 microns. The
density of the boron nitride pellets formed from the cold pressing
operation is no greater than an average of about 1.80 g/cc or 80%
of the theoretical density of hexagonal boron nitride independent
of the number of repeated granulation and cold pressing stages.
In the conversion of high purity hexagonal boron nitride to cubic
boron nitride the compacts or pellets of boron nitride formed by
compaction are subjected to extremely high pressures and
temperatures within the stable region of the cubic boron nitride
phase diagram. The density of the boron nitride pellets is
significant to the economics of the cubic boron nitride conversion
process.
It has been discovered in accordance with the present invention
that the density of cold pressed boron nitride powder may be
substantially increased to a density of at least about 1.86 g/cc
and approximating 1.9 g/cc i.e approximating 85% of theoretical by
controlling the particle size distribution of the boron nitride
particles prior to compaction so that the distribution of particle
sizes is as wide as possible and preferably with the majority of
the particles having a particle size above 50 microns. The
preferred particle size range for the majority of the particles
should be between 20-500 microns. It has been further discovered in
accordance with the present invention that agglomerated particles
of boron nitride formed from a wide boron nitride particle size
distribution following cold press compaction and granulation will
possess a density closer to the average density achieved with hot
pressing. In addition its thermal conductivity for use as a filler
is enhanced particularly for use as a filler material in polymer
composites.
SUMMARY OF THE INVENTION
The method of the present invention broadly comprises the steps of
forming high purity hexagonal boron nitride; crushing said high
purity hexagonal boron nitride into boron nitride particles
extending in size over a size range of at least 100 microns with
the majority of the particles having a particle size above 50
microns, cold pressing the crushed particles into a compacted form
and granulating the compacted particles into a granulated powder
with the operations of cold pressing and granulation occurring in
one or more stages.
Agglomerated boron nitride particles of high density and high
thermal conductivity are also formed from cold pressed hexagonal
boron nitride in accordance with the present invention by the
process of crushing high purity hexagonal boron nitride into boron
nitride particles extending in size over a size range of at least
100 microns with the majority of the particles having a particle
size above 50 microns, cold pressing the crushed particles into a
compacted form, granulating the compacted form into a granulated
powder, with the cold pressing and granulation steps occurring in
one or more stages, to a suitable size for use as fillers of high
thermally conductivity.
BRIEF DESCRIPTION OF THE DRAWINGS
The advantages of the present invention will become apparent from
the following detailed description of the present invention when
read in conjunction with the accompanying drawings of which:
FIGS. 1A and 1B show four schematic flow diagrams labeled A,B,C,
and D comparing the standard prior art method of compaction labeled
A for cold pressing high purity boron nitride with the alternate
methods of the present invention labeled B.C., and D
respectively;
FIG. 2 shows typical particle size distribution curves for the high
purity boron nitride particles formed from the milling operation
used in the standard compaction method A of FIGS. 1A and 1B;
FIG. 3 shows a corresponding particle size distribution curve for
the high purity boron nitride particles formed from crushing high
purity boron in each of the alternate methods B, C and D shown in
FIGS. 1A and 1B; and
FIG. 4 is a graph showing the relationship of density to the powder
compaction cycle i.e. number of powder compaction stages for milled
and crushed powder.
DETAILED DESCRIPTION OF THE INVENTION
The schematic flow diagram labeled A in FIG. 1 (FIGS. 1A and 1B)
illustrates the current practice of forming a compact of cold
pressed boron nitride starting from high purity boron nitride
material. The high purity boron nitride material is converted into
a very fine high purity hexagonal boron nitride (hBN) powder using
a conventional milling operation utilizing, e.g., a high speed
impact mill to reduce the BN to a fine powder. Typical properties
of the fine high purity BN powder are shown below in Table I.
TABLE 1 ______________________________________ Properties of Milled
High Purity BN & Final Compacted Density (After 2 Cycles)
Sample 1 2 3 4 ______________________________________ % Oxygen
0.605 0.488 0.275 0.548 Surface Area (m.sup.2 /g) 5.67 5.48 5.26
6.39 Tap Density (g/cc) 0.58 0.60 0.51 .47 % Soluble Borates 0.33
0.16 0.10 .26 -325 Mesh Sizing 99.9 99.9 99.9 99.9 Avg. Part. Size
(.mu.m) 10.62 10.05 10.25 9.59 Max. Part. Size (.mu.m) 42.2 29.9
29.9 29.9 Density (g/cc) (2X) 1.80 1.79 1.77 1.81 % Theoretical
Density 80.0 79.6 78.7 80.44
______________________________________
As evident from the above Table 1 in conjunction with FIG. 1 column
A the fine powder formed from the milling operation has a particle
size distribution such that 99.9% is milled to below -325 mesh (44
micron) and has an average particle size of 5-11 microns and a
maximum particle size of no more than about 42 microns. The size of
this powder is difficult to handle and to compact. With the powder
being so fine, it needs to be deaired (prepressed) before
compaction. If it is not deaired, the powder tends to flow out of
the press die during compaction. Deairation is typically conducted
by pressing the powder at a relatively high pressure in a large
uniaxial press. The resultant cake is then granulated. This
feedstock is then loaded into the hopper of an automated uniaxial
press, for continuously pressing the powder into small pellets or
plugs of material of e.g., 2.25" diameter.times.2" long. After the
first pressing, the compacted boron nitride (BN) may again be
granulated and again compacted in the automated uniaxial press. In
fact the deairation/compaction/granulation procedure may be
repeated in any number of stages. As can be seen FIG. 4, the
highest green density achieved for a compact formed from milled
powder was 1.74 g/cc or 77.3% of theoretical. By repeating the
compaction/granulation steps the density increased slightly as
indicated in sample 4 in Table 1 to a density of 1.81 g/cc.
Additional stages of compaction/granulation will have little effect
on the density as is confirmed in FIG. 4. Also, from observation
the tops of the compacted pellets have a tendency to delaminate,
indicating springback after pressing. It was discovered that the
density of the compacted pellets may be increased to approximately
1.9 g/cc i.e. up to 85% of theoretical by controlling the
distribution of the boron nitride particle sizes before compaction
by extending the range of particle sizes to a range of over at
least 100 microns. The preferred method for achieving a wide
particle size distribution is to crush the hard chunks of high
purity BN resulting from the low purity boron nitride high
temperature treatment. This yields a wide distribution of particle
sizes as is evident in Table II shown below and in FIG. 3. This is
accomplished in accordance with the method of the present invention
by substituting a crushing operation for the milling operation as
is evident in FIG. 1 flow sequence B,C and D respectively. The
crushing operation as shown in flow diagram B may be followed by
the standard deairation/granulation-compaction/granulation stages
as used in the standard flow diagram A of FIG. 1 or only by
compaction/granulation stages without a deairation step as shown in
the flow diagram sequences C or D of FIG. 1. The final step of the
flow diagram sequence D of FIG. 1 is a crushing operation to
produce a wide particle size dispersion of agglomerated boron
nitride particles. This is particularly useful as a thermally
conductive filler material particularly to fill polymers.
TABLE II ______________________________________ Properties of
Crushed High Purity BN & Final Compacted Density (After 2
Cycles) Sample 1* 2 3 4 ______________________________________
Oxygen 0.571 0.275 0.426 .60 Surface Area (m.sup.2 /g) 3.16 5.26
2.51 3.02 Tap Density (g/cc) 0.76 0.85 0.92 .89 % Soluble Borates
-- 0.10 0.09 .14 Screen Sizing +40 -- 11.12 71.64 67.51 -40 +80 --
41.46 12.04 13.72 -80 +100 -- 6.16 1.08 1.82 -100 +150 -- 8.36 2.20
3.01 -150 +200 -- 5.76 1.44 2.42 -200 +325 -- 7.32 2.32 2.90 -325
11.16 19.82 9.28 8.62 Density (g/cc) (2X) 1.86 1.89 1.91 1.84 %
Theoretical Density 82.70 84.0 84.90 81.78
______________________________________ *Note: Sample 1 was crushed
and screened to remove the fines. Samples 2 and 3 were not.
The above Table II shows the typical properties for roll crushed
high purity boron nitride. Crushing of the high purity boron
nitride material leaves a much wider particle size distribution and
higher tap density, both of which are evident from table II result
in a higher compaction density. The tap density of the crushed
particles is at least 0.76 g/cc. The sample 1 lot was screened to
remove the fines whereas samples 2, 3 and 4 were not screened. The
tap density and precompacted density of Sample 1 is higher than the
milled samples but is less than samples 2, 3 and 4. The deairation
step was eliminated in Samples 1-3 of the roll crushed lots and a
uniaxial pressing, granulation, uniaxial pressing procedure as
described with the milled material was used for compaction. The
compacted density is significantly higher than in Table I. Also,
delamination did not occur with these samples, indicating less of a
tendency for springback. The higher compacted densities and lower
tendency for springback in the roll crushed material are useful for
conversion to cubic boron nitride. The flow diagram D in FIG. 1 may
be used to produce boron nitride crushed agglomerated particles as
high thermally conductive fillers.
The following are examples of the present invention:
EXAMPLE 1
Highly agglomerated high purity boron nitride powder was crushed in
a roll mill to the particle size distribution shown in FIG. 3 with
properties as shown in Table 2 column 3. This powder was was
compacted using a horizontal press at a pressure of 19,000 psi. The
compacted pieces were granulated by forcing the material through a
screen with openings approximately 1/2 inch. The granulated
particles were again compacted at 19,000 psi. The procedure is
outlined in FIG. 1 in the flow diagram labeleed C. The density of
the resultant compact was 1.91 g/cc. For comparison a highly
agglomerated high purity boron nitride powder was milled to a fine
powder using the standard milling procedure as shown in the flow
sequence A of FIG. 1. The properties of this powder Sample 4 are
shown in Table 1 column 4. The particle size distribution of this
powder is shown in FIG. 2 as the curve labeled "Milled Fine Powder
4". This fine powder was deaired at 2500 psi and granulated
followed again by compaction, granulation and compaction as
outlined in FIG. 1 flow diagram A. The density of the resultant
compacts were only 1.81 g/cc.
EXAMPLE 2
Highly agglomerated, high purity boron nitride powder was crushed
in a two roll mill to the particle size distribution as shown in
FIG. 3 labeled as "Crushed Powder 4". The properties of this powder
are shown in Table 2 column 4. This powder was compacted using a
horizontal press at a pressure of 19,000 psi. Four additional
granulation/compaction steps as described in example 1 were
performed. The resultant compact density was 1.91 g/cc. The density
of the compacts as a function of compaction cycles is shown on FIG.
4, on the curve labeled "Roll Crushed Powder 4". In comparison the
highly agglomerated high purity boron nitride powder was milled to
a fine powder. The properties of this powder are shown in Table 1
column 4. The particle size distribution of this powder is shown in
FIG. 2 as the curve labeled "Milled Fine Powder 4". This fine
powder was deaired at 2500 psi and granulated followed by the same
five cycles of granulation and compaction as was done for the
crushed powder. The density of the resultant compacts was only 1.84
g/cc. The density of the compacts as a function of compaction
cycles is shown on FIG. 4 on the curve labeled "Prepressed+Milled
Powder 4".
EXAMPLE 3
Highly agglomerated, high purity boron nitride powder was crushed
in a roll mill to the particle size distribution shown on FIG. 3
labeled as "Crushed Powder 2". The properties of this powder are
shown in Table 2 column 2. This powder was compacted using a
horizontal press at a pressure of 19,000 psi. The compacted pieces
were granulated by forcing the material through a screen with
openings approximately 1/2 inch. The granulated particles were
again compacted at 19000 psi. The density of the resultant compact
was 1.89 g/cc. This should be compared with the four different fine
milled powders of Table 1 in which the resultant compact densities
are 1.80, 1.79, 1.77, and 1.81 g/cc respectively.
EXAMPLE 4
Highly agglomerated high purity hexagonal boron nitride powder with
the properties as indicated on Table 2 Column 4, was crushed using
a roll mill to the particle size distribution shown on FIG. 3 in
the curve labeled "Crushed Powder 4". This crushed powder was
compacted at 19,000 psi using a powder compaction press (uniaxial
press), and was then formed into granules of 1/16 inch and finer
using a granulator. The granules were once again compacted at
19,000 psi. These compacts were crushed using a sawtooth and roll
crusher and screened through a 120 mesh screen, resulting in a
powder that had a tap density of 0.68 m.sup.2 /g. The screened
particles were added to a cresol novalak thermoset epoxy
formulation with a phenol novalak hardener at a 65 wt % BN loading,
and then molded in a transfer press. The thermal conductivity of
the molded compound was 7.4 W/m.degree. C.
The pellets or compacts may again be crushed into a powder compound
of agglomerates having a wide particle size distribution. These
agglomerates are of higher density than corresponding agglomerates
formed from milled powder undergoing the same steps.
* * * * *